Unit 6 - Electric Circuits
6-1 Electric Current Electric Current Electric current is the flow of electric charge In a lightbulb light and heat are produced when electrons flow through the filament, and 'bump" into the atoms Electric Circuit An electric circuit is the path through which electrons flow Electrons cannot flow through an open circuit (circuit is not connected) Electrons can flow through an open circuit (circuit is connected) In an electric circuit electrons move in a chain reaction, pushing the electrons in adjacent atoms forwards through the circuit Formula Electrons move quite slowly (mm/s) even though the effect of turning on a circuit happens very fast Current is measured in amperes (A), the rate at which amperes flow through a certain point 1A=1C/s (1 ampere = 1 coulomb per second) (1 coulomb = 6.24x1018) I=q/t (current = charge / time) Parts of a Circuit Mandatory Components power source conductors (wire) load Optional switch fuse - prevents to much current from flowing through - when too much current flows through the fuse melts creating an open circuit Conventional vs Electron-Flow Current NOTE: This is proof that IB is intentionally trying to screw with your mind by teaching you one thing but making you do another Conventional current is more used in workplaces and by professors, it says that electrons flow from positive to negative, but it's WRONG Electron-flow is less used however it is CORRECT as it says that electrons flow from negative to positive NOTE: ON ALL IB EXAMS OR EVALUATIONS USE CONVENTIONAL NOTE: If you are consistent you can use either option as they would both work NOTE: The reason the majority of people use the wrong method is because it is the one that came first AC and DC Current DC - direct current - continuous flow of charge in one direction - battery AC - alternating current - direction of current alternates back and forth but the energy flows like a wave - outlet AC is used for large scale production because it is cheaper and easier to transmit over large distances Calculating Voltage, Current and Resistance in Series and Parallel Circuits NOTE: This section will consist of part from 6-1, 2 and 3 The formula V=IR (voltage = current x resistance) will be used quite often in this unit, so you better know it :) Voltage Voltage is what causes current to flow in a circuit Look at voltage like water which always flows from higher gravitational potential energy to lower gravitational energy Voltage flows from higher electrical potential energy to lower potential energy Batteries store chemicals that create different levels of electrical potential energy at the ends, this difference is called potential difference Voltage is the work done per unit charge to move a small positive charge between two points V=E/q (voltage = energy(work) / charge) Resistance Resistance is the force that opposes the flow of current Resistance is measured in Ohms R=pl/A (resistance (Ohms) = resistivity of material (Ohms x m) x length of material(m) / cross sectional area (m2)) If a circuit is broken the air will act as a resistor of infinite resistance thus stopping electron flow Series Circuits There are no junctions in a series circuit and the current has only one path to follow (if this explanation is not enough to explain to you what a series circuit looks like maybe you should do the unthinkable and google it) A series circuit has the same current in all parts of the circuit The sum of the voltages across all the loads in a series equals to the voltage across the battery The V=IR formula can be used to calculate the resistance of each individual load The total resistance of the circuit can be found by simply adding up the resistances Parallel Circuits In a parallel circuit there are multiple ways for electrons to get from one side of the power source to the other (once more google it if this information is not enough to paint an image) The voltage across two branches that split at the same point and join at the same point is the same The amperage at a point before a junction is the same as that where the branches meet The total resistance within a single branch of a circuit can be calculated as you would a series circuit The total resistance in a parallel circuit is: ((RTOTAL)-1=(RTOTAL OF BRANCH 1)-1+(RTOTAL OF BRANCH 2)-1+(RTOTAL OF BRANCH 3)-1+...+(RTOTAL OF BRANCH n)-1 Voltmeters and Ammeters Voltmeters Voltmeters measure volts(potential difference) Compares energy of electrons at two places in a circuit Must be placed in parallel with the circuit Ideal voltmeters have infinite resistance A non-ideal voltmeter can be drawn with internal resistance in parallel with an ideal voltmeter Ammeters Measure current flowing through a circuit Coulombs of electrons passing a point every second Must be placed in series with a circuit Ideal ammeters have zero resistance A non-ideal ammeters can be drawn with internal resistance in series with an ideal ammeter Power NOTE: This is from 6-3 Power is measured in watts P=IxV (Power = current x voltage)(my time is too precious to derive this and show you that it is infact true so then just simply go with it) More branches in a circuit means more current is drawn, more current means more power, more power and the battery dies faster Batteries An ideal battery has no internal resistance A non-ideal battery can be drawn with internal resistance in series with an ideal cell Terminal Voltage the voltage across the end of the battery (VT). Terminal Voltage is higher when there are no loads attached EMF the voltage of a battery without internal resistance (Vemf) Vemf=Ix(Ri+Re) (EMF=current x (internal resistance + external resistance) Vemf=IxRi+VT (EMF=current x internal resistance + terminal voltage) Components A battery has three components (cathode, anode, electrolyte) Cathode - positive electrode - undergoes a reaction that produces positive ions Anode - negative electrode - undergoes reaction that produces negative ions Electrolyte - transports ions between the two side Types of Batteries Disposable - primary cells - alkaline batteries Rechargeable - secondary cells - nickle-cadmium - lithium ion - lead acid NOTE: In order to recharge a battery, a current must be sent opposite to the direction of travel when discharging the battery Potentiometers A potentiometer is a variable resistor If the potentiometer is set at half then the its resistance would be half (I know mind blowing)